The development of improved extraction methods for shale gas and natural gas are providing an abundant source of hydrocarbon feedstocks. These feedstocks are gas mixtures that typically contain 80-99% methane, 1-20% ethane, 1-5% higher hydrocarbons and other non-hydrocarbon constituents, such as CO2 and nitrogen. While the exploitation of wet shale gas is ramping up and new ethane crackers are being built, the large fraction of methane from shale gas today is mainly used as fuel or as a feedstock for syngas production.
Converting the methane directly into chemical products presents a technical challenge. The oxidative coupling of methane (OCM) is one of the most explored routes for the direct conversion of methane into products. The OCM reaction to ethylene is: 2 CH4+O2→C2H4+2H2O. Despite the fact that intense research was dedicated to OCM catalyst development, the yields of ethylene and other desired C2+ hydrocarbon molecules appear to have reached a maximum of about 20-30%, suggesting a limit. The main issue with the OCM process is that oxygen is more reactive with the products than with methane. As the concentration of products increases, so does the rate of side reactions. Since the OCM reaction occurs at high temperatures, e.g., 400-1000° C., the choice of the catalyst has little effect on the speed of the side reaction, and the majority of methane is non-selectively oxidized to carbon dioxide. The second main issue with OCM is the costly separation of the ethylene out of the OCM product stream, which contains unreacted methane, H2, CO, H2O, CO2 and C2+ hydrocarbons such as, but not limited to, ethylene and ethane. The composition of the product of an OCM process not only depends on the catalyst type, but also on the type of oxygen source, such as air or pure oxygen, and operating conditions (CH4/O2 ratio, P, T, contact time, and reactor type). The influence of catalyst type, operating conditions and cofeed options on the product composition of the OCM process is known to those skilled in the art.
It would be desirable to have a process that could take advantage of the increased supply of shale gas to convert it into useful chemical products while avoiding the costly separation of ethylene from the product stream.